Silver halides have been widely used as the light sensitive components in photographic elements. Upon exposure of the silver halides to light, a latent image is formed which is then developed during the photographic processing to form a visible image. Silver halides are intrinsically sensitive only to light in the blue region of the visible spectrum. To impart to silver halides sensitivity to other wavelengths of radiation, visible such as green or red as well as invisible such as infrared, spectral sensitizing dyes, such as cyanine dyes, are used in photographic elements. Said sensitizing dyes, adsorbed to the surface of silver halide grains, absorb light or radiation of a particular wavelength and transfer the absorbed energy to the silver halide to form a latent image.
Dyes which have been capable of sensitizing silver halides to infrared regions of the electromagnetic spectrum have been known for many years. Cyanine and merocyanine dyes, particularly those with longer bridging groups between cyclic moieties have been used for many years to sensitize silver halide to the infrared. U.S. Pat. Nos. 3,619,154; 3,682,630; 2,895,955; 3,482,978; 3,758,461; 4,515,888 and 2,734,900, and GB Pat. Nos. 1,192,234 and 1,188,784 disclose well-known classes of dyes which sensitize silver halide to portions of the infrared region of the electromagnetic spectrum.
With the advent of lasers, and particularly solid state laser diodes emitting in the infared region of the electromagnetic spectrum, the interest in infrared sensitive photographic elements has greatly increased. These infrared emitting diodes have a wide variety of emission wavelengths, the most interesting ranging from about 700 to 900 nm. Typical emission wavelengths include 750 nm, 810 nm, 820 nm, and 870 nm. Many different processes and articles useful with laser diodes have been proposed for a number of applications, such as for making prints from computer assisted tomography and various graphic arts products.
With the increasing popularity of infrared emitting laser diodes as exposure sources for photographic elements, it is desiderable to provide silver halide materials offering an increased exposure latitude upon exposure to said diodes, and higher maximum density upon photographic processing.
The following is a description of the prior art, cited with reference to the present invention.
Silica (silicon dioxide) has been widely disclosed for use as matting agent in photographic elements, as described for example in U.S. Pat. Nos. 3,411,907; 4,409,322; 4,499,179; in EP Pat. Applications Nos. 395,956 and 404,091 and in Japanese Pat. Applications Nos. 62005-235 and 60188-942. Typically, silica used as matting agent has grain size of 0.1 micrometers or more. U.S. Pat. No. 4,711,838 describes silver halide photographic materials for laser exposure to near infrared containing, in a top coat layer and/or a backing layer, surface roughening agents (including silica) having average particle sizes in the range from 0.1 to 1.5 micrometers, to prevent formation of non-contact interference fringes.
Colloidal silica (i.e., silica having grain size below 0.1 micrometers) has been widely described as antistatic compound for use in photographic elements comprising a photosensitive layer and an antistatic layer coated on the film base on the side opposite to a that of the photosensitive layer, as described for example in U.S. Pat. No. 3,525,621 and in EP Pat. Applications 296,656 and 334,400.
Colloidal silica has been also widely disclosed for use in protective layers of photographic elements for reducing scratching, glossiness or adhesion, as described for example in U.S. Pat. Nos. 4,232,117; 4,264,719; 4,777,113 and 4,985,394 and in Japanese Pat. Application No. 03168-637.